CF4 Glow Discharge Modification of CH4 Plasma Polymer Layers

Introduction. Low-pressure, non-isothermal glow discharges can be used to deposit polymeric layers at ambient temperatures. Plasma polymerization occu...
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Langmuir 1996, 12, 4205-4210

4205

CF4 Glow Discharge Modification of CH4 Plasma Polymer Layers Deposited onto Asymmetric Polysulfone Gas Separation Membranes J. Hopkins and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England, U.K. Received November 21, 1995. In Final Form: April 11, 1996X

Post CF4 glow discharge modification of methane plasma polymer layers deposited onto asymmetric polysulfone membranes has been investigated by XPS, FTIR, AFM, and gas permeability measurements. Oxygen and nitrogen gas permeability and permselectivity through the polysulfone substrate are found to be strongly influenced by the plasma processing parameters.

Introduction Low-pressure, non-isothermal glow discharges can be used to deposit polymeric layers at ambient temperatures. Plasma polymerization occurs via the excitation of precursor molecules by an electrical discharge; this leads to monomer fragmentation and the formation of a variety of reactive intermediates including radicals and ions which may subsequently undergo recombinational collisions to form growing oligomeric species either in the gas phase or at the surface of an adjacent substrate.1 The resultant material can be chemically and physically different in nature compared to polymers prepared by conventional polymerization techniques. Take methane for example; it is a cheap and stable molecule, which readily undergoes glow discharge polymerization to produce cross-linked, pinhole-free hydrocarbon films.2-4 Methane plasma polymer films have found application as barrier coatings in biomedical systems,2 for improving the adhesive bonding of inert surfaces3 (e.g. PTFE or stainless steel), and also in the field of gas separation membranes.2 In the latter case, plasma polymerization of methane/fluoromonomer gas mixtures has been shown to generate compact polymeric networks, which can enhance the gas separation performance of the host polymer membrane substrate.5 In this study, rather than using a methane/fluoromonomer gas mixture, a pure methane plasma polymer layer is post-treated by a CF4 glow discharge, with the aim of improving the O2/N2 gas separation characteristics of asymmetric polysulfone membranes. The CF4 glow discharge treated methane plasma polymer films have been characterized as a function of electrical discharge power by X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and gas permeability measurements. Experimental Section Asymmetric polysulfone membranes were prepared by casting polysulfone (Udel P3500, Union Carbide) 20% by weight dissolved in dimethylacetamide (99%+, Lancaster) onto a glass substrate * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Yasuda, H. Plasma Polymerisation; Academic Press: New York, 1985. (2) Ho, C.; Yasuda, H. J. Appl. Polym. Sci. 1990, 39, 1541. (3) Inagaki, N.; Yasuda, H. J. Appl. Polym. Sci. 1981, 26, 3333. (4) Yasuda, H.; Sharma, A.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1385. (5) Inagaki, N.; Ohkubo, J. J. Membr. Sci. 1986, 27, 63.

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at ambient temperature, followed by immersion in deionized water. The resultant membrane was then ultrasonically washed in an isopropyl alcohol/hexane mixture for 30 s and dried in air. High-purity carbon tetrafluoride (99.7%, Air Products) and methane (99.7%, Air Products) gases were used for the respective plasma treatments. Glow discharge experiments were carried out in a cylindrical glass reactor (4.5 cm diameter, 515 cm3 volume, base pressure of 1.5 × 10-3 mbar, and with a leak rate better than 2.0 × 10-3 cm3 min-1) enclosed in a Faraday cage.6 This was fitted with a gas inlet, a Pirani pressure gauge, and a 27 L min-1 two-stage rotary pump attached to a liquid nitrogen cold trap. A matching network was used to inductively couple a copper coil (4 mm diameter, 13 turns, spanning 9-18 cm from the gas inlet) wound around the reactor to a 13.56 MHz radio frequency (rf) source. All joints were grease-free. Gas flow and leak rates were calculated by assuming ideal gas behavior.7 A typical experimental run comprised initially scrubbing the reactor with detergent, rinsing with isopropyl alcohol, and oven drying; this was followed by a 60 min high-power (50 W) air plasma cleaning treatment. Next, the reactor was opened up to the atmosphere, a strip of polymer was inserted into the center of the rf coils, and then the system was evacuated back down to its original base pressure of 1.5 × 10-3 mbar. Subsequently CH4 gas was introduced into the reaction chamber at a pressure of 2 × 10-1 mbar and a flow rate of approximately 1.9 cm3 min-1 (i.e. at least 99.6% of the reactor contents). The reaction zone was purged with CH4 for 5 min, and then the glow discharge was ignited for 5 min, followed by the methane feed gas being allowed to continue to flow through the reactor for a further 5 min. Next, the plasma system was evacuated back down to its base pressure prior to introducing CF4 gas into the reaction vessel at a pressure of 2 × 10-1 mbar. After allowing 5 min for purging, the CF4 plasma was ignited for 5 min. Upon completion of treatment, the RF generator was switched off and the reactor flushed with CF4 gas for a further 5 min prior to venting to atmospheric pressure. Each sample was characterized immediately after electrical discharge treatment by X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and gas permeability measurements. A Kratos ES300 electron spectrometer equipped with a Mg KR X-ray source (1253.6 eV) and a concentric hemispherical analyzer was used for XPS surface analysis. Photoemitted core level electrons were collected at a takeoff angle of 30° from the substrate normal, with electron detection in the fixed retarding ratio (FRR, 22:1) mode. XPS spectra were accumulated on an interfaced PC computer and curve fitted using a Marquardt minimization algorithm. Instrument performance was calibrated with respect to the gold 4f7/2 peak at 83.8 eV with a full-width(6) Shard, A. G.; Munro, H. S.; Badyal, J. P. S. Polym. Commun. 1991, 31, 152. (7) Ehrlich, C. D.; Basford, J. A. J. Vac. Sci. Technol. 1992, A10, 1.

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at-half-maximum (fwhm) of 1.2 eV. Photoelectron ionization cross-section sensitivity factors for unit stoichiometry were taken as being C(1s):O(1s):S(2p):N(1s):F(1s) equals 1.00:0.55:0.54:0.74: 0.53. XPS was used to check for the cleanliness of the polysulfone substrate and for the absence of any surface-active inorganic additives. Gross and experimental errors were calculated for each surface treatment. FTIR analysis involved essentially the same type of experimental procedure as described above, except a KBr disk was employed as the substrate and the plasma treatments were carried out over a longer period of time in order to improve sensitivity, typically 30 min for methane plasma polymerization and 15 min for CF4 glow discharge treatment. A FTIR Mattson Polaris instrument was used to acquire 100 scans at a resolution of 4 cm-1. Atomic force microscopy offers structural characterization of surfaces in the 10-4 to 10-10 m range without the prerequisite of special sample preparation (e.g. metalization). Applications to date have included imaging on the atomic scale of insulator surfaces,8 elucidation of the surface microstructure of soft polymers,9-11 and dialysis/filtration membranes.12,13 A Digital Instruments Nanoscope III atomic force microscope was used to examine the topographical nature of the polysulfone substrate prior to and following electrical discharge exposure. All of the AFM images were acquired in air using the Tapping mode14 and are presented as unfiltered data. The probe scanning direction was carefully chosen in order to avoid tip-induced artifacts. Mass spectrometric sampling devices15 have been previously used to evaluate the permeability of common elastomers.16,17 In this study, methane plasma polymer was deposited onto the dense side of the asymmetric polysulfone membrane and placed into a permeability probe.18 This consisted of two drilled-out stainless steel flanges with a copper gasket to ensure a leak-tight seal, and an electron microscope grid to provide mechanical support to the membrane. The probe was coupled to the sample preparation chamber of a Kratos ES300 X-ray photoelectron spectrometer (base pressure 2 × 10-10 mbar) by means of a stainless steel tube inserted through a rotatable gate valve. The coated side of each sample film was exposed to a gas pressure of 800 mbar. High-purity nitrogen (BOC, 99.995%) and oxygen (BOC, 99.6%) gases were used. Permeant pressure was monitored with a UHV ion gauge (Vacuum Generators, VIG 24). A quadrupole mass spectrometer (Vacuum Generators SX200) interfaced to a PC computer was used to analyze the chemical composition of the permeant species. The quadrupole mass spectrometer’s response per unit pressure was calculated by introducing each gas in turn into the chamber and recording the corresponding mass spectrum at a predetermined pressure of 5 × 10-6 mbar (taking into account ion gauge sensitivity factors19). The mean equilibrium permeant partial pressure (MEPPP) of each gas was measured in the steady-state flow regime.20 (8) Heinzelmann, H.; Meyer, E.; Gruetter, P.; Hidber, H. R.; Rosenthaler, L. H.; Guentherodt, J. J. Vac. Sci. Technol. 1988, A6, 275. (9) Zajac, G. W.; Patterson, M. Q.; Burrell, P.; Metaxas, M. C. Ultramicroscopy 1992, 42-44, 998. (10) Overney, R. M.; Luethi, R.; Haefke, H.; Frommer, J.; Meyer, E.; Guentherodt, H. J.; Hild, S.; Fuhrmann, J. Appl. Surf. Sci. 1993, 64, 197. (11) Schoenherr, H.; Snetivy, D.; Vancso, G. J. Polym. Bull. 1993, 30, 567. (12) Kasper, K.; Herrmann, K. H.; Dietz, P.; Hansma, P. K.; Inacker, O.; Lehmann, H. D.; Rintelen, T. H. Ultramicroscopy 1992, 42-44, 1181. (13) Chahboun, A.; Coratger, R.; Ajustron, F.; Beauvillain, J.; Aimar, P.; Sanchez, V. Ultramicroscopy 1992, 41, 235. (14) Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. Lett. 1993, 290, L688. (15) Westover, L. B.; Tou, J. C.; Mark, J. H. Anal. Chem. 1974, 46, 568. (16) Laurenson, L.; Dennis, N. T. M. J. Vac. Sci. Technol. 1985, A3, 1707. (17) Tou, J. C.; Rulf, D. C.; DeLassus, P. T. Anal. Chem. 1990, 62, 592. (18) Barker, C. P.; Kochem, K.-H.; Revell, K. M.; Kelly, R. S. A.; Badyal, J. P. S. Thin Solid Films 1995, 259, 46. (19) Pressure measurement technical information, Vacuum Generators Limited. (20) Crank, J.; Park, G. S. In Diffusion in Polymers; Crank, J., Park, G. S., Eds.; Academic Press: London, 1968; Chapter 1.

Figure 1. C(1s) XPS spectra: (a) clean polysulfone; (b) CH4 plasma polymer coated polysulfone (20 W, 5 min); (c) CF4 plasma treatment of (b) at 10 W, 5 min; NB the two components at lowest binding energy correspond to Mg KR3,4 satellite peaks with different fwhm.

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Results (a) XPS. The XPS spectrum of clean untreated polymer substrate was consistent with the parent structure of bisphenol-A-polysulfone (PSF):21

The C(1s) XPS envelope can be fitted to carbon environments corresponding to carbon bound to hydrogen at 285.0 eV, carbon bound to sulfur at 285.6 eV, and carbon singly bound to oxygen (ether linkage) at 286.6 eV (Figure 1a). The O(1s) XPS spectrum displayed an unresolved 1:1 doublet at 532.1 and 533.6 eV, whilst the S(2p3/2, 1/2) spectrum showed an unresolved 2:1 doublet centred at 168 eV. Both the O(1s) and S(2p3/2, 1/2) XPS peak shapes are consistent with the known chemical structure of bisphenol-A-polysulfone.21 Plasma polymerization of methane produced a hydrocarbon layer containing trace amounts of oxygen and nitrogen (less than 2%); these impurities probably arise from trapped free radical centers at the surface reacting with the atmosphere during sample transfer into the XPS spectrometer.22,23 Complete coverage by CH4 plasma polymer of the polysulfone substrate was signified by the absence of any S(2p3/2, 1/2) signal showing through. A peak at 285.0 eV in the C(1s) region was consistent with the formation of CxHy centers (Figure 1b). No variation in the C(1s) envelope was found over the 5-50 W power range. This is not surprising, since core level XPS is unable to detect hydrogen. CF4 plasma treatment of the deposited plasma polymer at 10 W results in a dramatic change of the C(1s) envelope, with fluorinated moieties becoming the predominant groups:21,24 12.8 ( 3.2% CF3 (293.3 eV), 31.9 ( 3.1% CF2 (291.2 eV), 16.9 ( 3% CFn-CFn (289.3 eV), 9.1 ( 0.3% CF (287.8 eV), and 8.1 ( 0.6% C-CFn (286.6 eV) (Figure 1c). This gives rise to a F:C ratio of 1.08 ( 0.05. Once again no sulfur is observed, thereby indicating that the methane plasma polymer layer has not been etched away by the CF4 glow discharge treatment.25 Variation of the input power (5-50 W) during methane plasma polymerization, whilst maintaining the CF4 discharge power at 10 W produced no change in the elemental composition at the surface nor in the relative distribution of fluorinated carbon centers. Similarly, the CF4 discharge power level (10-50 W) did not appear to influence the level of surface fluorination (when the power of the methane glow discharge was kept constant at 20 W). (b) FTIR. Methane plasma polymerization gave rise to the deposition of a hydrocarbon layer with corresponding FTIR absorbances at23,26,27 2950 cm-1 (asymmetric CH3 stretch), 2928 cm-1 (CH2 asymmetric stretch), 2870 cm-1 (CH3 symmetric stretch), a broad peak centered around (21) High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Beamson, G., Briggs, D., Eds.; Wiley: New York, 1992. (22) Inagaki, N.; Kawai, H. J. Polym. Sci., Polym. Chem. Ed. 1986, 24, 3381. (23) Sharma, A. K.; Yasuda, H. J. Appl. Polym. Sci. 1989, 38, 741. (24) Wells, R. K.; Ryan, M. E.; Badyal, J. P. S. J. Phys. Chem. 1993, 97, 12879. (25) Hopkins, J.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 4261. (26) Silverstein, R. M.; Clayton Bassler, G.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1991; Chapter 3. (27) Shinohara, H.; Iwasaki, M.; Tsuyimara, S.; Wantanabe, K.; Okazaki, S. J. Polym. Sci. 1972, A1, 2129.

Figure 2. FTIR spectra: (a) CH4 plasma polymer deposited onto KBr (20 W, 30 min); (b) CF4 plasma treatment of (a) at 10 W, 15 min.

Figure 3. AFM micrographs of asymmetric polysulfone membrane: (a) dense side; (b) porous side.

1637 cm-1(CdC stretch), 1460 cm-1 (CH2 wagging or scissoring), and 1377 cm-1 (CH3 symmetric bending) (Figure 2a). These infrared absorbances displayed no variation with increasing electrical discharge power (1050 W); similar findings have been previously reported.28 There was an additional weak absorbance band at 2349 cm-1 due to background CO2 gas impurities present in the FTIR spectrometer during data acquisition. CF4 plasma treatment of the hydrocarbon plasma polymer layer produced a slight enhancement of the broad feature in the 1600-1700 cm-1 region; this ovelaps with the following FTIR absorbances:29 1626 cm-1 (sCFdC< (28) El Hossary, F. M.; Fabian, D. F.; Webb, A. P. Thin Solid Films 1990, 192, 201. (29) Ryan, M. E.; Fonseca, J. L. C.; Tasker, S.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 7060.

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Figure 4. AFM micrographs: (a) 5 W CH4 plasma polymer layer deposited for 30 min onto the dense side of an asymmetric polysulfone membrane; (b) following 10 W CF4 plasma treatment of (a); (c) 50 W CH4 plasma polymer layer deposited for 30 min onto the dense side of an aysmmetric polysulfone membrane; (d) following 10 W CF4 plasma treatment of (c).

Figure 5. Variation in CH4 plasma polymer particle size with glow discharge power.

stretch in a cross-linked environment) and 1730 cm-1 (sCFdCFs stretch) (Figure 2b). A difference in the depth of analysis accounts for the much more dramatic change evident in the XPS spectra compared to the FTIR data.30 Clearly, fluorination must only be occurring within the outer surface region of the methane plasma polymer layer. (c) AFM. A compact structure is evident on the dense face of the untreated asymmetric polysulfone membrane, whilst 1.27 ( 0.16 µm pore apertures are discernible on the opposite face (Figure 3). Plasma polymerization of methane produced a globular surface texture. An increase in methane glow discharge power causes the average particle size to decrease, i.e. become more densely packed (Figures 4 and 5). Post-treatment of the methane plasma polymer with a CF4 glow discharge resulted in the unveiling of a much (30) Walls, J. M., Ed. Methods of Surface Analysis; Cambridge University Press: New York, 1989.

better defined globular surface texture, again reflecting the decrease in particle size with increasing glow discharge power (Figure 4). (d) Gas Permeability Measurements. An O2/N2 permselectivity of 3.09 ( 1.02 was measured for the untreated asymmetric polysulfone membrane. This is in fairly good agreement with literature values31 of 4.7. Both the N2 and O2 mean equilibrium permeant partial pressures (MEPPPs) increase with glow discharge power used during CH4 plasma polymerization onto the dense side of an asymmetric polysulfone membrane, whilst the O2/N2 permselectivity is approximately halved with respect to the unmodified substrate for powers greater than 5 W (Figure 6). CF4 plasma treatment at 10 W for 15 min of a range of CH4 plasma polymers deposited onto the dense side of the asymmetric membrane causes a decrease in permeant pressures relative to the untreated asymmetric membrane and just CF4 glow discharge modification of the asymmetric membrane. This is accompanied by a shift in O2/ N2 permselectivity back toward that of the untreated polysulfone membrane (Figure 7). Discussion There are numerous reactive intermediates which can be formed during the electrical discharge excitation of methane;32,33 these include CH3, CH2, CH, and ionic species (31) Fritzche, A. K.; Murphy, M.; Cruse, C. A.; Malon, R. F.; Kesting, R. E. Gas Sep. Purif. 1989, 3, 106. (32) Tachibana, K.; Harima, H.; Wano, Y. J. Phys. D: Appl. Phys. 1984, 17, 1727. (33) Kline, L. E.; Partlow, W. D.; Beis, W. E. J. Appl. Phys. 1985, 65, 70.

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Figure 7. Gas permeation of 10 W CF4 plasma treated methane plasma polymer layer deposited at increasing powers onto the dense side of an asymmetric polysulfone membrane: (a, top) gas permeability; (b, bottom) O2/N2 selectivity. Figure 6. Gas permeation of CH4 plasma polymer layer deposited for 30 min onto the dense side of an asymmetric polysulfone membrane: (a, top) gas permeability; (b, bottom) O2/N2 selectivity.

such as CH+. Depending upon the experimental conditions employed, either diamond-like-carbon or polymeric hydrocarbon materials can be deposited.3,4,34 In the latter case, the CH3 radical is regarded as being the chief reactive intermediate.33 The amorphous hydrocarbon films produced in this study were found to readily undergo fluorination by a CF4 glow discharge. CF4 is considered to be a nonpolymerizable gas which does not form plasma polymer material; instead it leads to the grafting of fluorine moieties onto polymeric surfaces.35-37 This can be understood in terms of constituent fluorine atoms in the CF4 plasma25,38-40 undergoing hydrogen abstraction and substitution reactions at the hydrocarbon plasma polymer surface to produce mCF, >CF2, and -CF3 functionalities.41 The presence of the mCF and tertiary carbon centers at the plasma polymer surface suggest a highly cross-linked network, whilst -CF3 moieties signify chain ends.34 (34) Wang, J.; Feing, D.; Wang, H.; Rembold, M.; Thommen, F. J. Appl. Polym. Sci. 1993, 50, 585. (35) Plasma Deposition, Treatment and Etching of Polymers; d’Agostino, R., Ed.; Academic Press: New York, 1990; Chapter 2. (36) Plasma Deposition, Treatment and Etching of Polymers; d’Agostino, R., Ed.; Academic Press: New York, 1990; Chapter 1. (37) Cramarossos, F.; DeBenedictus, S. Plasma Chem. Plasma Process. 1982, 2, 213. (38) Strobel, M.; Corn, S.; Lyons, C. S.; Korba, G. A. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 1125. (39) Kay, E. Proc. Int. Ion Congr. ISIAT & IPAT, 1983. (40) Cain, S. R.; Egitto, F. D.; Emmi, F. J. Vac. Sci. Technol. 1987, A5, 1578. (41) Iriyama, Y.; Yasuda, H. J. Polym. Sci., Polym. Chem. Ed. 1992, 30, 1731.

Clearly the depth of fluorination is fairly thin (